Would love to hear from [MENTION=100539]Mellemel[/MENTION] about what the problem & solution was. I'm apparently having the same symptoms as well. All of a sudden, my coolant levels were abysmally low - I'd had to put probably 3/4 to 1/2 gallon to refill the radiator and the reservoir. Then I'd drive, keeping my eyes on the OBD readings for coolant temp, where it would stay constant at about 190F for 100mi (climbing up to Lake Tahoe, about 7000ft elevation gain (if that makes a diff). After stopping, I'd open up the hood to check on the reservoir - looked the same as before. No weird exhaust, no leaks onto the ground. Then overnight, I'll check the engine bay again in the morning, then lo and behold, the reservoir level went from right between the high and low marks, to the lowest end of the low mark -losing probably around 250ml, or half a pint. I'd check the ground again to find no leaks.
Furthermore, in my case:
2000 4runner 6cyl
no white exhaust
dipstick & oil cap looks not milky
nothing dripping onto the ground
radiator cap looks good
any pointers would be greatly appreciated